1. Field of the Invention
The present invention relates to a carrier for developing electrostatic latent images for use in electrophotographic methods and electrostatic recording methods, a two-component developer and an image forming method using the carrier.
2. Description of the Related Art
In electrophotographic image formation, an electrostatic latent image is formed on an image bearing member comprising a photoconductive material, and the electrostatic latent image is developed into a toner image with a charged toner. The toner image is then transferred onto and fixed on a recording medium. In the field of electrophotography, full-color copiers and printers have been brought to the mainstream in place of monochrome copiers and printers recently.
In a typical full-color image formation, toner layers of yellow, magenta, cyan, and optional black are superimposed on one another to reproduce various colors, and the resulting composite toner image is finally fixed on a recording medium.
Conventionally, one-component developing methods, two-component developing methods and hybrid developing methods are used. In order to produce clear full-color images having good color reproducibility, a toner amount on an electrostatic latent image bearer needs maintaining faithfully to an electrostatic latent image, and the two-component developing methods are used in many cases for apparatuses required to produce high quality images at high speed.
When the toner amount on an electrostatic latent image bearer varies, the image density varies on a recording medium or color tone varies. The toner amount on an electrostatic latent image bearer varies because the toner charge quantity or the developer resistivity varies.
As the developer is used, a toner is spent on the surface of a carrier and occasionally varies in charge quantity. In this case, a resin having high water repellency is used in a coated layer of the carrier to prevent the toner from being spent, and Japanese published unexamined application No. JP-H11-202630-A discloses a method of adding a new carrier in a developer and discharge the spent carrier to prevent variation of charge quantity. Toner is likely to be spent in high density printing when a large amount of the toners are replaced.
A resin coated on a carrier is occasionally chipped by stirring stress of an image developer. When the resin coated layer is chipped and a core material is exposed, the carrier deteriorates in resistivity and image density deteriorates. In order to prevent the resin coated layer from being chipped, the resin coated layer is thickened or a large amount of fillers are mixed therein to increase strength thereof. A resin coated on a carrier is likely to be chipped when the number of printed sheets per 1 job is low because stirring time in an image developer becomes longer per one sheet, and in low-density printing because the carrier is not fed much.
As the filler mixed in the resin coated layer to control the resistivity of the carrier, an electroconductive material is used. Carbon black is mostly used as the electroconductive material. However, chipped resin coated layer mixes in color images, resulting in possible color contamination.
Japanese published unexamined application No. JP-2006-163368-A discloses a method of including a filler having an electroconductive coated layer formed of a tin dioxide layer and an indium oxide layer including tin dioxide in a resin coated layer to prevent the color contamination due to the chipped resin coated layer.
However, the filler surface resistivity is likely to increase after the indium oxide layer is worn out, resulting in high resistivity of the carrier. When a carrier has high resistivity, the developability deteriorates and the image density deteriorates. Thin lines and edges are highlighted in images, and when the toner concentration is increased to cover the deterioration of the image density, the toner is likely to scatter and cause background fouling.
Because of these reasons, a need exist for a carrier preventing deterioration of resistivity due to exposition of the core material even when the number of printed sheets per 1 job is low and in low-density printing, increase of resistivity due to chipped electroconductive layer of a filler included in a resin coated layer, and toner spent even in high-density printing.